Forest giants (4)
I believe the answer is:
oaks
'forest giants' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oaks that I've seen before include "Epsom Classic race" , "Hardwood trees" , "Mighty ones from little acorns grow" , "Horse race - trees" , "Trees of the genus Quercus" .)